This invention relates to a face insert for a golf clubhead, and, more particularly, to a face insert having a variable hardness.
Golf clubheads fall into two general categories--wood-type clubheads and iron-type clubheads. In the past wood-type clubheads have usually been made of wood, but in recent years wood-type clubs have predominantly been made from metal. As used herein, "wood-type" clubheads include both wood and metal clubheads.
A golf ball generally travels straightest when it is struck by a golf clubhead when the face of the clubhead is square to the intended line of flight of the ball and the center of gravity of the clubhead is aligned with the ball. When the ball is struck off-center, i.e., toward the toe or the heel of the clubhead, the ball tends to travel away from the intended line of flight.
When an iron clubhead strikes a ball on the toe portion of the clubhead, the ball tends to be projected to the right of the line of flight. When the ball is contacted by the heel of the clubhead, the ball tends to be projected to the left of the intended line of flight. Balls contacted above center tend to go up, and balls contacted below center tend to go down.
Conversely, when a ball is contacted by the toe of a wood-type of clubhead, the ball tends to curve to the left because of the gear effect of the wood clubhead. When a ball is contacted by the heel of a wood type of clubhead, the ball tends to curve to the right because of gear effect. Balls contacted above center tend to go down and balls contacted below center tend to go up. See U.S. Pat. Nos. 4,420,156 and 4,471,961 for a discussion of gear effect. In order to counteract or compensate for gear effect, the face of a wood-type clubhead is generally provided with bulge and roll curvatures as described in U.S. Pat. Nos. 4,420,156 and 4,471,961.
Face inserts have been used for many years on wooden clubheads. Face inserts are now approved for metal clubheads by the Rules of Golf of the Untied States Golf Association. However, the face inserts have generally been formed from uniform material and have not been constructed to compensate for off-center hits.